When comparing FlashDevelop vs WebStorm, the Slant community recommends WebStorm for most people. In the question“What are the best programming text editors?”WebStorm is ranked 16th while FlashDevelop is ranked 34th. The most important reason people chose WebStorm is:

WebStorm integrates with Node.js to allow for running, debugging, and unit testing of Node.js apps.

Pros

Pro

Free

Pro

Good code completion

Pro

Haxe development support

FlashDevelop has first-class support for Haxe development, the open source toolkit based on a modern programming language and cross-platform library.

Pro

Excellent support for Actionscript 2 and 3 (Flash)

Although everyone claims Flash is dead, it's still quite useful for game developers due to its rapid compile and run times, as well as its great debugging functionality with FlashDevelop.

Pro

Good number of project templates

While it's project template system is not the best compared to it's competitors, it still is decent and is a good way to generate some boilerplate code.

Pro

XML/HTML completion

FlashDevelop has XML/HTML completion aside from code completion.

Pro

.NET Framework 2.0 application

It's windows only, but has tremendous support from plugin developers and a dedicated team that's been developing it for close to 10 years.

Pro

Source-control support (svn, git, mercurial)

Pro

Great debugging

FlashDevelop provides very efficient debugging features.

Pro

Supports Zen-coding for HTML

This is very useful for carrying out high-speed HTML coding and editing.

Pro

Snippets

Pro

Tasks/todo

Pro

SWF/SWC exploration

Pro

Great project compilation

FlashDevelop facilitates project compilation.

Pro

Decent code generation

Although the code generation can't really be called top-notch, it's decent and sufficient for most developers.

Pro

Node.js integration

WebStorm integrates with Node.js to allow for running, debugging, and unit testing of Node.js apps.

Pro

Code refactoring support

To make sure your code can be easily maintained, you must first be sure to make it clean and tidy. This is the part where WebStorm really helps you. It automatically refactors your code by performing functions on it, such as extraction of variables, moving files, inline variable extraction, etc.

Pro

Integrated code quality tools

WebStorm comes bundled with JSHint and JSLint. JSCS, ESLint, and Closure Linter can be installed via npm. They register as inspections and are customizable through IDE settings. They run automatically and will highlight potential issues. Pressing alt+enter on an issue will allow the user to view suggested fixes.

Pro

Intelligent code completion

WebStorm has two types of autocompletion: structural completion and word expansion.

Structural autocompletion makes predictions based on its understanding of JavaScript objects, while the latter tries to predict the word currently being typed based on previously typed words. Word expansion also works in comments and docstrings and it's similar to vim's omnicompletion.

Both types of autocompletion work extremely well, have little to no problems and are quite fast even when loading suggestions on the go.

Pro

Built-in web server

WebStorm has a built-in webserver that allows you to run projects from http://localhost:63342/ProjectName.

Pro

Supports a wide range of plug-ins

Pro

Maintains a local history

The local history feature in Webstorm tracks all your local changes in the source code that you are making. You can use it to view changes that you've made to your code and roll back whenever necessary.

Pro

Typescript support

WebStorm offers typescript support for building typed JavaScript applications which improves refactoring and code completion, as well as helping to find bugs.

Pro

Reduces the amount of repetitive code that has to be written with code snippets

WebStorm includes a feature called Live Templates. Live Templates are predefined code snippets that can include variables. They're intended to increase productivity by reducing the amount of repetitive code that has to be written.

Pro

Ability to base hot keys on previously used IDEs

When you install WebStorm, it allows you to choose from other IDE's for it to base it's hot keys on. So if you are switching from another IDE, it makes it a very easy transition and productivity is not adversely affected by having to learn new hot keys.

Pro

Supports many JavaScript frameworks

WebStorm includes support for Meteor, Angular, and React.

Pro

Powerful git and GitHub integration

WebStorm has a powerful visual git tool, allowing for easy commits, visual diffing, merging, push/pull, rebasing, and inspecting the VCS history of a project. GitHub is supported natively - you can check out a project directly from GitHub.

Pro

Compilers for compile-to-JavaScript languages

WebStorm has built-in support for CoffeeScript and Dart.

Pro

Support for Karma test runner

Karma allows you to execute unit tests and monitor code coverage. You can read more about using Karma with WebStorm here.

Pro

Built-in support for JavaScript task runners

WebStorm has support for Grunt and Gulp task runners.

Pro

Built-in terminal/run tools

WebStorm (and really all of the IntelliJ IDEs) support the plugins throughout their plugin ecosystem which leaves you with 100s of tools to handle your automation tasks. There is a wide range of build-related plugins that help you by having pre-defined commands to execute with the click of a button. Out of any other IDE, WebStorm has by far the most coverage when it comes to tools for your development workflow.

Pro

CSS pre-processor support

WebStorm has built-in support for Sass which is one of the most stable and powerful CSS extension languages.

Pro

Server and client-side debugger

WebStorm has a powerful debugger, with support for conditional breakpoints.

Pro

Theme support / tweaking

The theme is very easy to customize to your liking. You can change font-size, colors, highlighting colors, and more.

Pro

Support for all major VCS systems

Webstorm supports not just Git and Mercurial, but Subversion, CVS, and Perforce as well.

Pro

Multi-line select and editing

Pro

Really good configurable code formatting

This integrates with other community tools like ESLint and editorconfig.

Pro

Powerful Code Templates

Pro

Special icons for most filetypes in project list

Webstorm comes with icons for many filetypes which makes it easier to find what you're looking for in the project list.

Pro

Free for open-source development, students, and teacher

Non-commercial open source projects, and students and teachers (including classroom licenses) may qualify for a free license. There is a 50% off license for startups (under 3 years old). You can read more about it here.

Pro

Has a built-in terminal

The IDE comes with a built-in terminal, a feature lacking in some lighter editors.

Con

Haxe debugging is in its infancy

Con

Occasional slow performance

WebStorm can sometimes choke all cores of the CPU. There are numerous reports of high CPU usage.

Con

Not free for commercial use

A paid license is required to use WebStorm for commercial use. The license terms changed in November 2015 and currently require a subscription (per year: $59 personal, $129 for companies). Students can obtain a free non-commercial, educational license good for one year. There is also an option to pay on a monthly basis in addition to perks, such as a fallback commercial license that can be used for free.

Con

Not open source

This application is proprietary, and thus cannot be modified, freely distributed, or trusted for security.

Con

You may have to fiddle with the JVM memory settings in configuration files

To get it to run properly, you have to edit the JVM memory settings when you use Node.js. Node.js makes the small JS project you have into a "large" project that requires more memory.

Con

Plug-in Ecosystem isn't robust

Every framework or extension with any popularity whatsoever will have a plugin for VS Code. Sometimes they'll still support Atom. Only one in twenty will have native support for WebStorm.

If you want support for the latest libraries, you won't get it in WebStorm.

Con

Proprietary file dialogs

Webstorm doesn't use the native Windows and OSX file dialogs, which makes it more of a hassle to open projects.For instance, you don't have access to your favorite folders on the left-hand side in their custom file dialogs.

Con

Non-native filesystem causes issues

The Java wrapper around the filesystem doesn't actively watch for file changes (by, for example, using the fsevents api on OS X), and as a result can become easily desynchronised from the actual filesystem.

If you have an external tool acting on your project (such as a gulp task or a third-party Git client), what you see in the file browser or in open tabs becomes out-of-date. There's a feature in the context-menu for manually synchronising directories with their real filesystem equivalent, but this shouldn't be necessary and is annoying to do. You usually remember to do that anyway after you've been trying to track down a bug on a line of JavaScript that Webstorm says doesn't exist for the last two hours.

It should be noted though that this is easily remedied by going to File/Settings/System Settings and checking the "Synchronize Files on frame or editor tab activation" option. It's also recommended to more explicitly represent your workflow within WebStorm itself. Most external tools/tasks can be handled with WebStorm. And if it's not in a plug-in, then you can handle it with the File Watchers.

Con

Poor usability on option and menu navigation

You need to press a combination of keys or navigate through different menu levels to carry out the two most common things a developer does. For example, to run your code, the default is Shift+F10. To go to definition, you need to click on "Navigation" first. There is no sense to this: being forced to press a combination of keys hundreds of times or navigating through different menus can be a waste of time.

Con

Can't open multiple projects in a single instance

The only way to open multiple projects is to run a new instance of WebStorm which is not ideal. WebStorm can use up to 1-1.5 GB of memory.

Con

Constantly trailing behind on support for its main features

The Jetbrains team do an admirable job attempting to keep up with support for the features they wrap, but they are running an un-winnable race. For example, WebStorm 11.0.2 hangs when trying to debug NodeJS 5.0+ projects (30+ seconds before hitting breakpoint). By the time they fix it, 5.1 will be out and something else will break.

Con

Newer versions doesn't respect `require`

Version 11 dropped support of require in favor of import autocomplete. This causes you to learn how to use it again, but most users wont have time for that.